1. Field of the Invention
The present invention relates to a carrier having a core material particle, a surface of the core material particle being covered with a covering layer.
2. Description of the Related Art
In image formation by an electrophotographic method, latent electrostatic images are formed on a latent electrostatic image bearing member such as those made of a photoconductive material, toner images are formed by making charged toners adhere to these latent electrostatic images, and then the resulting toner images are transferred to a recording medium, and fixed to form output images. In recent years, technologies such as copiers and printers using electrophotographic methods are gradually developing from those using monochromatic electrophotographic methods to those using full-color electrophotographic methods, and the rate of development is increasing. Thus, the market for technologies using full-color electrophotographic methods is increasing.
In full-color image formation, generally, three color toners of yellow, magenta, and cyan, or four color toners of black as well as yellow, magenta, and cyan are laminated to reproduce all colors. Thus, in order to obtain a sharp full-color image excellent in color-reproducibility, it is necessary to smooth the surface of a fixed toner image so as to reduce light scattering. Accordingly, image gloss obtained by means of conventional full-color copiers, and the like, is often middle to high, that is, 10% to 50%.
Generally, as a method of fixing a dry-toner image on a recording medium, the contact-heating fixing method is often employed in which a roller or belt having a smooth surface is press-contact with toner while heating the roller or belt. This method has advantages in that it exhibits high-thermal efficiency, enables high-speed fixing and enables toners used in polychromatic image formation to have glossiness and transparency. On the other hand, this method inconveniently causes a so-called offset phenomenon in which a part of a toner image adheres to the surface of a fixing roller and then transferred onto another image, because a surface of a heat-fixing member is made in contact with a molten toner under application of pressure and then they separates from each other.
With a view to preventing the offset phenomenon, there has been carried out a method of forming a surface layer of a fixing roller with use of a material excellent in releasing property, such as a silicone rubber and a fluorine resin, and further applying a toner adhesion preventing oil, such as a silicone oil, onto the surface layer of the fixing roller. Although this method is extremely effective in preventing toner-offset, this method requires additionally providing a device for supplying the oil, leading to upsizing of the fixing device.
As a result, in monochromatic image formation, an oil-less system in which a toner, which has a high viscoelasticity at the time of melting and contains a releasing agent in order to avoid internal fracture of the molten toner, is used to dispense with oil application onto a fixing roller, or a system in which the toner is used to make the application amount of oil very small is increasingly used.
Meanwhile, also in full-color image formation, an oil-less system tends to be employed for downsizing of a fixing device and simplification of the structure as well as in monochromatic image formation. However, in full-color image formation there is a need to smooth the surface of a fixed toner image, and thus to reduce the viscoelasticity of the toner in a molten state. Therefore, the full-color image formation can more easily cause the offset phenomenon than the monochromatic image formation, which produces less glossiness, and the full-color image formation becomes more difficult to employ the oil-less system. When a releasing agent is incorporated into a toner, the adhesive strength of the toner is increased, so that the transferability of the toner to a recording medium is degraded. Further, the incorporation of the releasing agent into a toner disadvantageously causes toner filming, leading to degradation in chargeability and then in durability.
Meanwhile, a carrier, a surface of which is coated with a silicone resin, is known as being a carrier designed to meet objectives, such as to prevent toner filming, form a uniform surface, prevent surface oxidation and degradation of moisture sensitivity, extend a lifetime of a developer, prevent adhesion onto the surface of a photoconductor, protect the photoconductor from being scratched and wearing, control charge polarity, and adjust the charge amount.
Conventionally known in the art is a carrier composed of at least a carrier core material particle, a surface of which being covered with a covering layer produced by a condensation using a titanium catalyst between (i) a polysiloxane material and (ii) a coating liquid containing a siloxane-based material having one of a silanol group and a precursor group thereof (e.g., a hydrolysable group, such as a halosilyl group and an alkoxysilyl group) which is capable of undergoing the condensation. For example, Japanese Patent Application Laid-Open (JP-A) No. 2001-92189 discloses a carrier composed of at least a core material particle, a surface of which is coated with a silicone resin containing an organic titanium-based catalyst, and lists for an example of titanium-based catalyst diisopropoxy bis(acetyl acetonate) titanium (described herein in Comparative Example 2) as a chemical which has the same effect as tetraisopropoxy titanium (described herein in Comparative Example 1), isopropoxy (2-ethyl hexanediolato) titanium, bis(acryloyloxy) isopropoxy isostearoyloxy titanium, bis(2,4-pentadionato) (1,3-propanedionato) titanium, or the like. JP-A No. 06-222621 discloses a carrier composed of at least a core material particle, a surface of which is coated with a coating agent having as a main component a coating composition consisting of at least an organopolysiloxane, organosilane, and a curing catalyst which is at least one selected from the group consisting of titanium (for example, the above mentioned tetraisopropoxy titanium), tin (for example, dibutyl tin diacetate as described herein in Comparative Example 3), zinc, cobalt, iron, an aluminum-based compound, and amines. Furthermore, JP-A No. 2006-337828 discloses a carrier composed of at least a core material particle, a surface of which is coated with one of a silicone resin and a modified silicone resin which contains at least a quaternary ammonium salt catalyst, an aluminum catalyst, or a titanium catalyst (specifically the above-mentioned diisopropoxy bis(acetyl acetonate) titanium).
However, these carriers disadvantageously cause blocking therewithin at the time of forming a covering layer, and the durability thereof is degraded.